COMPARISON THEOREMS IN RIEMANNIAN GEOMETRY JEFF CHEEGER DAVID G. EBIN AMS CHELSEA PUBLISHING !MERICAN -ATHEMATICAL 3OCIETY s 0ROVIDENCE 2HODE )SLAND COMPARISON THEOREMS IN RIEMANNIAN GEOMETRY David G. Ebin and Jeff Cheeger G.EbinandJeff David Photo courtesy of Ann Billingsley COMPARISON THEOREMS IN RIEMANNIAN GEOMETRY JEFF CHEEGER DAVID G. EBIN AMS CHELSEA PUBLISHING American Mathematical Society • Providence, Rhode Island M THE ATI A CA M L ΤΡΗΤΟΣ ΜΗ N ΕΙΣΙΤΩ S A O C C I I R E E T ΑΓΕΩΜΕ Y M A F O 8 U 88 NDED 1 2000 Mathematics Subject Classification. Primary 53C20; Secondary 58E10. For additional information and updates on this book, visit www.ams.org/bookpages/chel-365 Library of Congress Cataloging-in-Publication Data Cheeger, Jeff. Comparison theorems in riemannian geometry / Jeff Cheeger, David G. Ebin. p. cm. — (AMS Chelsea Publishing) Originally published: Amsterdam : North-Holland Pub. Co. ; New York : American Elsevier Pub. Co., 1975, in series: North-Holland mathematical library ; v. 9. Includes bibliographical references and index. ISBN 978-0-8218-4417-5 (alk. paper) 1. Geometry, Riemannian. 2. Riemannian manifolds. I. Ebin, D. G. II. American Mathe- matical Society. III. Title. QA649 .C47 2008 516.373—dc22 2007052113 Copying and reprinting. Individual readers of this publication, and nonprofit libraries acting for them, are permitted to make fair use of the material, such as to copy a chapter for use in teaching or research. Permission is granted to quote brief passages from this publication in reviews, provided the customary acknowledgment of the source is given. Republication, systematic copying, or multiple reproduction of any material in this publication is permitted only under license from the American Mathematical Society. Requests for such permission should be addressed to the Acquisitions Department, American Mathematical Society, 201 Charles Street, Providence, Rhode Island 02904-2294, USA. Requests can also be made by e-mail to [email protected]. c 1975 held by the American Mathematical Society. All rights reserved. Reprinted by the American Mathematical Society, 2008 Printed in the United States of America. ∞ The paper used in this book is acid-free and falls within the guidelines established to ensure permanence and durability. Visit the AMS home page at http://www.ams.org/ 10987654321 131211100908 Contents Preface to the AMS Chelsea Printing vii Preface ix Chapter 1. Basic Concepts and Results 1 1. Notation and preliminaries 1 2. First variation of arc length 3 3. Exponential map and normal coordinates 6 4. The Hopf-Rinow Theorem 9 5. The curvature tensor and Jacobi fields 11 6. Conjugate points 14 7. Second variation of arc length 16 8. Submanifolds and the second fundamental form 18 9. Basic index lemmas 20 10. Ricci curvature and Myers’ and Bonnet’s Theorems 23 11. Rauch Comparison Theorem 24 12. The Cartan-Hadamard Theorem 30 13. The Cartan-Ambrose-Hicks Theorem 31 14. Spaces of constant curvature 34 Chapter 2. Toponogov’s Theorem 35 Chapter 3. Homogeneous Spaces 47 Chapter 4. Morse Theory 69 Chapter 5. Closed Geodesics and the Cut Locus 81 Chapter 6. The Sphere Theorem and its Generalizations 93 Chapter 7. The Differentiable Sphere Theorem 105 Chapter 8. Complete Manifolds of Nonnegative Curvature 119 Chapter 9. Compact Manifolds of Nonpositive Curvature 137 Bibliography 149 Additional Bibliography 155 Index 157 v Preface to the AMS Chelsea Printing In the period since the first edition of this book appeared (1976), Rie- mannian geometry has experienced explosive growth. This circumstance was ultimately the determining factor in our decision not to try to bring the book up to date. We came to realize that if such an attempt were made, the book would never see the light of day. So our book remains essentially as it was. We hope that it will continue to play a useful role by setting out some (still relevant) fundamentals of the subject. There do exist a number of valuable surveys and expositions of many of the developments that have taken place in the intervening years. Some of these are indicated in a relatively small collection of additional references that have been added to the bibliography. The surveys of Berger, in partic- ular, provide a sweeping overview and include very extensive bibliographies. We have also added a highly selective (and quite inadequate) set of references to original sources, reflecting what we see as some of the most significant developments over the past 30 years. We apologize in advance to anyone whose work has been unjustly slighted. We are indebted to Satyaki Dutta for having retyped the manuscript and for a substantial amount of proofreading. We would also like to express our thanks to all those colleagues who, in the 20 years since the first edition went out of print, have urged us to consider having it republished. Jeff Cheeger and David G. Ebin, 2007. vii Preface In this book we study complete Riemannian manifolds by developing techniques for comparing the geometry of a general manifold M with that of a simply connected model space of constant curvature MH .Atypical conclusion is that M retains particular geometrical properties of the model space under the assumption that its sectional curvature KM , is bounded between suitable constants. Once this has been established, it is usually possible to conclude that M retains topological properties of MH as well. The distinction between strict and weak bounds on KM is important, since this may reflect the difference between the geometry of say the sphere and that of Euclidean space. However, it is often the case that a conclusion which becomes false when one relaxes the condition of strict inequality to weak inequality can be shown to fail only under very special circumstances. Results of this nature, which are known as rigidity theorems, generally re- quire a delicate global argument. Here are some examples which will be treatedinmoredetailinChapter8. Topological Theorem. If M is a complete manifold such that KM ≥ δ>0, then M has finite fundamental group. Geometrical Antecedent. If M is a complete manifold such that√ KM ≥ δ>0, then the diameter of its universal covering space M is ≤ Π/ δ. In particular, M is compact. Even if we assume M to be compact, the preceding statements are false if only KM ≥ 0. However, we can show the following. Rigid Topological Theorem. Let M be a compact manifold such that KM ≥ 0. Then there is an exact sequence 0 → Φ → Π1(M) → B → 0 where is a finite group and B is a crystallographic group on Rk for some k ≤ dim M, and therefore satisfies an exact sequence 0 → Zk → B → Ψ → 0 where Ψ is a finite group. Rigid Geometrical Antecedent. Let M be a compact manifold such that KM ≥ 0.ThenM splits isometrically as M × Rk (same k as above), where M is compact and Rk has its standard flat metric. Thus, if KM ≥ 0, M may not be compact, but it is at worst the isometric product of a compact ix xPREFACE manifold and Euclidean space. The infinite part of Π1(M) comes precisely from the Euclidean factor. The reader of this book should have a basic knowledge of differential ge- ometry and algebraic topology, at least the equivalent of a one term course is each. Our purpose is to provide him with a fairly direct route to some interesting geometrical theorems, without his becoming bogged down in a detailed study of connections and tensors. In keeping with this approach, we have limited ourselves primarily to those techniques which arise as out- growths of the second variation formula and to some extent of Morse theory. In Chapter 1 we have included a rapid treatment of the more elemen- tary material on which the later chapters are based. Of course, we do not recommend that the less knowledgeable reader regard this as a comprehen- sive introduction to Riemannian geometry. Likewise, Chapters 3 and 4 are provided in part for the convenience of the reader. In Chapter 3 which deals with homogeneous spaces, we also summarize without proof the rel- evant material on Lie groups. In Chapter 4 the main theorems of Morse theory are stated, again mostly without proofs. An exception, however, is Lemma 4.11, which is perhaps less standard than the other material. For the unproven results in both chapters, excellent references are readily available. Our main geometrical tools, the Rauch Comparison Theorems and the more global Toponogov Theorem, are discussed in Chapters 1 and 2 respectively. Chapter 5 deals with closed geodesics and the injectivity radius of the expo- nential map. Chapters 6-9 form the core of our study. Chapter 6 contains the Sphere Theorem – M simply connected and 1 ≥ KM > 1/4 implies M homeomorphic to a sphere – as well as Berger’s rigidity theorem which covers the case 1 ≥ KM ≥ 1/4. The last three chapters deal with material of recent origin. Chapter 7 is primarily concerned with the differentiable version of the Sphere Theorem. Chapter 8 takes up the structure theory of complete noncompact manifolds of nonnegative curvature, while Chap- ter 9 gives some results on the fundamental group of compact manifolds of nonpositive curvature. It is a pleasure to thank Carole Alberghine and Lois Cheeger for their patient work, typing the original manuscript. Bibliography M. BERGER [1959] Vari´et´es Riemanniennes `a courbure positive, Bull. Soc. Math. France 87. 1 [1960] Les vari´et´es Riemanniennes 4 -pinc´ees, Ann. Scuola Norm. Sup. Pisa (III) 14(2). [1960] Sur quelques vari´et´es Riemanniennes suffisamment pinc´ees, Bull. Soc. Math. France 88. [1961] Les vari´et´es Riemanniennes homog`enes normales simplement connexes `a courbure strictment positive, Ann. Scuola Norm. Sup. Pisa (III) 15(3).